Pure quantum systems can undergo phase transitions analogous to the classical phase transition between the liquid and gaseous states of water. At the quantum level, however, the particle spins in states that emerge from phase transitions display collective entangled behavior. This unexpected observation offers a new avenue for the production of materials with topological properties that are useful in spintronics applications and quantum computing.
The discovery was made by an international collaboration led by Julio Larrea, a professor at the University of São Paulo's Physics Institute (IF-USP) in Brazil. Larrea is first author of an article on the study published in Nature.
"We obtained the first experimental evidence of a first-order quantum phase transition in a quasi-two-dimensional system consisting entirely of spins. It was a groundbreaking study in terms of both experimental development and theoretical interpretation," Larrea said.
To understand the significance of this discovery, it will help to examine the classical phase transition, which can be exemplified by the change in the state of water, and its quantum analog, exemplified by the Mott metal-insulator transition.
"The change in the state of water, which occurs at 100 °C under standard atmospheric pressure, is what we call a first-order transition. It is characterized by a discontinuous jump in molecule density. In other words, the number of water molecules per unit volume varies drastically between one state and the other," Larrea said. "This first-order discontinuous transition evolves in accordance with pressure and temperature until it is fully suppressed at the so-called critical point of water, which occurs at 374 °C and 221 bar. At the critical point, the transition is second-order, i.e. continuous."
In the vicinity of the critical point, the properties of water behave anomalously, because the density fluctuations are infinitely correlated on the atomic length scale. As a result, the material manifests a unique state that differs both from a gas and a liquid…